if: guard against if_ioctl being NULL

There are situations where an struct ifnet has a NULL if_ioctl pointer.

For example, e6000sw creates such struct ifnets for each of its ports so it can
call into the MII code.

If there is then a link state event this calls do_link_state_change()
-> rtnl_handle_ifevent() -> dump_iface() -> get_operstate() ->
get_operstate_ether(). That wants to know if the link is up or down, so it tries
to ioctl(SIOCGIFMEDIA), which doesn't go well if if_ioctl is NULL.

Guard against this, and return EOPNOTSUPP.

PR: 275920
MFC ater: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")